In customer generation systems for generating electric energy such as solar generators, wind energy systems with PM generator, variable speed combustion engines or fuel cells, the power fed into the grid varies strongly. Attractive special rates are often offered for such customer generation systems so that, from an economical point of view, it is increasingly interesting for the operator of such systems to feed the generated power into the utility grid. Due to the different rates, the generated and consumed energy is usually metered using different meters. Grid parallel operation calls for taking into consideration particular conditions of security and connection.
What is referred to as network monitoring is also known; it is very important. For a customer generation system must detect the grid failure and then stop operating. It is not allowed to further feed power into the grid because of the possible risk of injury to grid maintenance workers when a grid section has been disconnected whilst the customer generation system continues to work in this section. Network monitoring methods are known, which are divided into passive and active methods. Passive methods only evaluate the measured values of one-phase and three-phase grid voltage and grid frequency. Active methods are characterized in that power or voltage distortions affect the grid in such a manner that the grid parameters or the grid impedance can be inferred from the response. From current distortion one obtains a voltage response and vice-versa, from voltage distortion one obtains a current response.
Accordingly, a grid monitoring system must be effective in what is referred to as the grid operation. In what is referred to as island operation in which the connection to the utility grid is interrupted and only the loads of a defined previously known section are further supplied, it is known to turn off grid monitoring. This happens because the grid impedance during grid operation is much lower than the impedance in the island mode of operation. Additionally, in such an approach, the modular extensibility is facilitated since the grid impedance of the island network changes in the event of a modular extension. Faulty cut-off, in particular in the event of greater load steps, of the internal grid monitoring of the customer generation system occurring as a result of the changed grid parameters are advantageously avoided. The entire system thus becomes more stable, more specifically when operated as an island network.
Customer generation systems are known in which the electric loads continue to be supplied upon a grid failure. They need additional devices.
One additional device in this sense is a functional unit the operator must install in order to allow an existing customer generation system to work as a backup power supply system.
Advantageously, when more electric loads need to be supplied with backup power, the system can only be extended using modules added in the form of additional components without certain electric loads having to be associated with a certain functional unit of the additional device.
For an uninterruptible supply of the loads to be ensured, it is practical to always operate the backup power system in the same control mode both when it is in load operation, in which a utility grid is provided, and in unload operation, in which a utility grid is not provided, so that there is no need to interrupt the supply in order to switch over the controller. The same applies in the event the utility grid resumes operation.
When customer generation systems are installed on line taps, inadmissible high grid voltages may generate in this line tap when the infeed is high and the consumption low. The additional device may then be used to absorb energy and to accordingly lower the grid voltage. As a result, the grid can be stabilized so as to allow for the installation of customer generation systems even on line taps, which would not be possible otherwise.
On the other side, the grid voltage may reach inadmissible low values on these line taps when consumption is very high. Here also, the additional device can be used in order to supply power from the battery for the high consumption peaks. Accordingly, the grid voltage at the consumption node can be kept stable even on greater electric loads.
Grid parallel customer generation systems are known that can be additionally used as backup power supply systems.